Air pollution regulations require that when emergency vented hydrocarbons are burned, there be no emission of smoke as the hydrocarbon burns. Since, of the known hydrocarbons, methane alone burns in open air at flares without smoke production, the problem of smoke suppression in flare operation is demanding, since hydrocarbons other than methane must be vented as required by operations.
In the prior art of smokeless flaring of all hydrocarbons, a preferred form of flare design provides for injection of steam, under significant pressure, at or close to the point of initiation of the burning. A variety of reactions and conditions cause steam injection per se to be what is considered as the most effective method for suppression of smoking at the flare. But since at times, a supply of steam for smoke suppression is not available, other means for suppression of smoking have been devised and are now considered common knowledge by those versed in the art.
One such device is tubular in structure and has a blower or fan located at one end of the tube and a burner for the vented hydrocarbons at the other end of the tube. Air is delivered through the tube at significant velocity, by fan or blower and meets the hydrocarbon gases emerging from the burner in such manner that the turbulence created by the velocity of air flow, very greatly speeds the combustion reaction toward the suppression of smoke. Such flares are commonly operated with the primary air tube in the vertical direction with the fan or blower at the bottom end of the tube and the burner at the upper end of the tube.
When the vertically oriented tube is in the open air, the discharge of combustion gases is directly to the atmosphere, but because the quantity of air delivered by the fan or blower is less than the quantity required for complete burning of the vented hydrocarbons, air from the atmosphere is drawn into the combustion zone to supplement fan or blower air, and complete the required combustion reaction. For this reason, atmospheric air must be available immediately as the hydrocarbons begin to burn.
Open air burning of fuels, which makes atmospheric air available for complete hydrocarbon burning, has two great disadvantages. One is that the flame immediately begins unrestricted heat loss by radiation. The second is that in the open air there is virtually constant air movement, by breezes or winds, which further reduces flame temperature, according to the velocity of the air movement. Greater velocity will increase the heat loss from the flame to such a degree that the fuel may cease burning (as when a match is extinguished by blowing on it).
Since fuels burn according to the temperature, turbulence and time, and since as has been stated, wind action cools the flame resulting from burning, it is expedient to protect the burning zone from wind action to avoid temperature reduction in the flame. Maximum flame temperature produces best and most complete hydrocarbon burning. Therefore, if there is no wind flowing against the flame, minimal heat is thus lost from the flame and combustion can better be completed.
Complete combustion occurs where there is no emission of smoke or other products of incomplete combustion such as CO (which is toxic), H.sub.2, as well as CH.sub.2 O (which is an irritant as well as toxic). Smoking is by far the most predominant pollutant, and can be readily seen, while gaseous pollutants, which typically are not a problem, cannot be seen. It is evident therefore that greatest concern is for avoidance of smoke as hydrocarbons burn.
The tendency for smoke production, that is, escape of unburned carbon as hydrocarbons burn, is a function of the weight-ratio of hydrogen to carbon (H/C) characteristic of the hydrocarbon, when there is no suppression of smoke. When the H/C ratio is 0.33, such as for methane, there is no smoke production. When the H/C ratio is 0.25 smoke production begins and as the H/C ratio falls lower, there is increased production of smoke such that with an H/C ratio of 0.166 (ethylene) the smoke is very dense. All of this is for the case where there is no suppression of smoke.
Smoke can be suppressed by increased turbulence in the burning zone, by air injection to the burning zone, and by high velocity injection of steam to the burning zone, and to combine air injection with increased turbulence by other means known to those versed in the art. However, the effectiveness of such smoke prevention measures is hindered if through wind action the temperature of the flame is decreased as is well known in the art.
In view of the discussion to this point, it would seem obvious to enclose the burning area for avoidance of wind action on the flame. But simple enclosure, per se, is not a solution because as earlier pointed out, there must be ready access of atmospheric air to the burning zone in quantities. Thus the problem of enclosure is burdened with not only access of atmospheric air to the burning zone, but supply of energy for movement of atmospheric air into the flame area for assured complete burning. Note here that when there is escape of carbon (black smoke) from the burning zone there is incomplete combustion of the hydrocarbon.
In typical natural draft fuel burning in furnaces, energy for air movement to the fuel for burning is supplied as draft by chimneys or stacks which, being filled with hot gases, supply draft energy at the stack base in keeping with the gas temperature and with stack height. As an example, a 100 foot high stack when filled with 1,200.degree. F. gases will supply static draft of approximately 1.0 inch WC (0.57 oz.) at its base. Burners are sized for the draft energy which is stack-supplied to assure ample air for fuel burning. Draft energy as supplied by stacks or chimneys is (as has been stated) not great and precaution must be taken to avoid upset in air delivery by draft when additional air is delivered by blowers or fans for the identical condition of fuels burning.